In liquid developers, fine grinding and dispersion of the toner particles is performed in a wet state, and therefore compared with dry powder toners, better micronization of the toner particles is possible. Further, because liquid developers use a carrier liquid containing an insulating liquid as the carrier, problems caused by scattering of the toner particles inside the image forming device do not arise. As a result, a feature of image forming devices that use a liquid developer is their ability to form high-definition images.
In electrophotographic image forming devices that use a liquid developer, a developer containing finely micronized toner particles dispersed in a carrier liquid is used. An electrostatic latent image formed on a photoreceptor by exposure is developed using the toner particles in the carrier liquid. Following developing, the obtained electrostatic latent image is transferred to a recording medium such as paper, and is then dried and fixed to complete image formation.
A liquid developer contains toner particles dispersed in an electrically insulating carrier liquid, and the toner particles require colorability, fixability, chargeability and dispersion stability. The toner particles contain a colorant, a binder resin and an additive such as a dispersant, and in order to obtain superior images, it is desirable that the toner particles can be dispersed stably and charged stably (for example, see Patent Literatures 1 and 2).
Various dispersants have been investigated with the aim of achieving uniform dispersion of the toner particles in the electrically insulating carrier liquid and superior storage stability. However, if the toner particles are dispersed stably and the storage stability is improved, then the amount of heat required in the fixation process during image output to achieve melting, contact and coalescence of the toner particles increases, and the fixability of the toner particles to the substrate deteriorates. Further, other problems also arise, including occurrence of a cold offset phenomenon in which some of the incompletely melted toner particles adhere to the surface of the thermocompression bonding roller and are subsequently transferred to the next sheet of paper. Accordingly, achieving a combination of good fixability and cold offset resistance with favorable storage stability has been difficult. Hence, in order to achieve a combination of favorable fixability and storage stability for a liquid developer, polymeric dispersants that are less likely to inhibit fixation have been investigated (for example, see Patent Literature 3).
However, the effects of such polymeric dispersants cause other problems, including a deterioration in the charging characteristics of the toner, meaning satisfactory image density cannot be obtained, and a deterioration in the long-term stability of the image, resulting in a deterioration in the color development and color reproducibility.